On the first 4 exams of her geometry class, Emily got an average score of 77. What does she need on the next exam to have an overall average of 80?
Explanation: Let her score on the next exam be $x$ The sum of all of her scores is then $4 \cdot 77 + x$ The same sum must also be equal to $5 \cdot 80$ Solve: $x = 5 \cdot 80 - 4 \cdot 77 = 92$.